1. Field of the Invention
The present invention relates to a method for nondestructively measuring each layer thickness of one or more thin film stacks stacked on a substrate.
2. Description of the Related Art
In the field of semiconductor devices and magnetic devices, for sophistication and performance improvement of elements, films to be formed have been made extremely thin and the number of the films to be stacked has also been increasing. Moreover, in a recent electronic device such as a semiconductor or magnetic device, control of film stack interfaces is also performed to control electron scattering at the film stack interfaces.
As a method for evaluating layer thicknesses of film stacks, ellipsometry and X-ray fluorescence analysis have heretofore been used. The ellipsometry is a method for obtaining a thickness and a refractive index of a sample thin film having a flat surface by making polarized light incident on the sample thin film and measuring a change in a polarized state of reflective light. However, since the ellipsometry uses light, the method has a problem that measurement cannot be performed unless the sample is transparent to light. The X-ray fluorescence analysis is a method for measuring fluorescence X-rays generated by a sample and estimating thicknesses on the basis of intensity of the X-rays. This method has the following problems. Specifically, only a total amount of elements generating the fluorescence X-rays is obtained and layer thicknesses are not directly measured. Moreover, when multiple thin films including the same elements are stacked, their layer thicknesses cannot be separately analyzed. Furthermore, neither the ellipsometry nor the X-ray fluorescence analysis can provide information on film stack interfaces.
Cross-section TEM observation for devices allows measurement of layer thicknesses of film stacks with very high spatial resolution and can also estimate interface widths. However, for the TEM observation, the sample needs to be divided into thin pieces of 100 nm or less, which leads to destructive analysis. Thus, although the TEM observation can be used for defect analysis, it is difficult to use the TEM observation for an inspection device.
X-ray reflectivity method is a method for nondestructively measuring layer thicknesses and interface widths of thin film stacks. The X-ray reflectivity method includes two kinds of methods. One is a method for making a monochromatic X-ray grazingly incident on a sample surface and measuring a reflectivity while changing an incident angle. The other one is a method for making a white X-ray incident on a sample and measuring wavelength dependence of a reflectivity. Both of the methods are for analyzing layer thicknesses by use of interference of the X-rays reflected from the sample surface and interface. Moreover, reflection from the surface or interface is influenced by the interface width. Thus, information on the interface widths of the film stacks can also be obtained by detailed analysis of an X-ray reflectivity profile. As a theoretical curve used for reflectivity analysis, used is a formula obtained by inserting the interface roughness effect of Sinha [Phys. Rev. B, 38, 2297 (1988) into an explicit formula of Parratt [Phys. Rev., 95, 359 (1954). Moreover, for Fourier transform analysis, often used is a method by Sakurai et al. [Jpn. J. Appl. Phys., 31, L113 (1992).